Online ruler in actual size
No ruler at hand? Hold a bank card or ID card to the screen once and match the size – after that the ruler measures in cm, mm or inches at actual size.
How it is calculated
Screens are packed differently: one centimetre on a phone contains far more pixels than on a large monitor. Browsers do not know the real size of their screen – so an online ruler needs a reference object. This ruler uses a card almost everyone carries.
How it works
- Tap “Calibrate with a card”. Hold a debit, credit, health insurance or ID card to the rectangle.
- Move the slider (or −/+) until card and rectangle match exactly, then tap “Save”.
- Choose a unit (cm, mm or inches), place the object at zero and tap the ruler – the marker shows the length.
Background: why a card?
Bank, credit and ID cards share the same worldwide format ID-1 from the standard ISO/IEC 7810: 85.60 × 53.98 mm. The International Civil Aviation Organization ICAO adopts these dimensions for identity documents (Doc 9303, Part 5, section 2.1). Since 1959 one inch has been exactly 25.4 mm (NIST). From the card width you set, the tool calculates how many pixels make one millimetre. Without calibration it assumes the CSS nominal value of 96 pixels per inch – which is rarely right on real screens, because according to the W3C a CSS pixel is a reference pixel, not a physical size.
Honest limits
- Accuracy depends entirely on your calibration – there is no guarantee. If the match is off by one percent, the ruler is off by one millimetre over 10 cm. For precision work a real ruler or caliper is more accurate.
- Browser zoom (Ctrl + / −) and changed display scaling alter the size – recalibrate afterwards. The value applies per device and browser.
- On a phone only the screen width fits on the ruler, so in portrait just a few centimetres. Measure longer items in sections.
- A printout of this page is not to scale – printers scale pages.
Privacy
The ruler is drawn in your browser. Only the calibration value (pixels per mm) is stored in the browser’s local storage; no camera is used.
Frequently asked questions
How accurate is an online ruler on a phone?
As accurate as your calibration: if the card match is 1 % too large, so are all measurements (1 mm over 10 cm). Without calibration the sizes are only approximate.
How do I show a ruler at actual size on my screen?
Calibrate once with a card in the standard card format (85.60 × 53.98 mm per ISO/IEC 7810). After that the ruler shows centimetres and millimetres at real size on exactly this device.
Can I measure cm and mm online?
Yes. Choose centimetres, millimetres or inches, place the object at zero and tap the ruler. The marker also shows the length in the other unit.
Which card can I use to calibrate?
Any card in ID-1 format: debit, credit, health insurance or loyalty card, or an ID card or driving licence in card format. All are 85.60 mm wide and 53.98 mm high.
Can I print the ruler?
Not to scale: printers and print dialogs scale the page. For a paper ruler, print a PDF template at “Actual size” (100 %) and check it with a real ruler.
Does the ruler measure with the phone camera?
No. It uses no camera and no sensors, only the screen display. AR apps measure with the camera, but they also only give approximate values.
Sources and legal basis
- ICAO Doc 9303, Part 5 – TD1 size: nominal dimensions per ISO/IEC 7810:2019 ID-1 (85.60 × 53.98 mm), section 2.1
- NIST – SI Units: Length (1 inch = 25.4 mm exactly)
- W3C – CSS Values and Units Level 3: absolute lengths (1in = 2.54cm = 96px)
- MDN Web Docs – Window.devicePixelRatio
As of:
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